(1) Field of the Invention:
The present invention relates to a process for producing silicon nitride ceramic articles, which can attain high strength at room temperature. More particularly, the invention relates to a process for producing silicon nitride ceramic articles, which is characterized by heating silicon nitride sintered products machined in a given shape in a temperature range from 550.degree. C. to 900.degree. C. in an oxidizing atmosphere.
(2) Background of the Invention:
In conventional techniques, silicon nitride ceramic articles have been produced by mixing a sintering aid and Si.sub.3 N.sub.4 powder, grinding and shaping the thus obtained mixture, and firing the shaped bodies to obtain sintered bodies, followed by (1) machining the sintered bodies, (2) further crystallizing them after the machining, or (3) crystallizing and then machining the sintered bodies. In such a case, the crystallization treatment is to increase strength at high temperature, and is effected to convert a glass phase in Si.sub.3 N.sub.4 sintered bodies to a crystalline phase.
However, the above-mentioned technique (1) in which only the machining is effected has a defect that machining scratches and cracks remain on surfaces of the silicon nitride ceramic articles to reduce strength at room temperature. To compensate such a defect, as described in Japanese patent application Laid-open No. 60-81,076, scratches and cracks formed on surfaces of sintered bodies by machining after sintering are removed by heating them in a temperature range from 950.degree. to 1,400.degree. C. However, it happens that their color and/or dimension changes, which may lead to deterioration in characteristics. In the technique (2) in which the crystallization is effected after the machining, there is a defect that although no reduction in bending strength at room temperature is seen, the technique cannot be applied to products necessitating high dimensional precision because a dimension slightly becomes smaller after the crystallizing treatment. On the other hand, the technique (3) in which the machining is effected after the crystallization has a defect that though this technique can be applied to products requiring high dimensional precision, machining scratches or cracks remain on surfaces thereof to lower strength at room temperature.
In Japanese patent application Laid-open Nos. 52-30,811, 58-79,885, and 61-178,472, it is reported that strength has been attempted to be improved by heating silicon nitride sintered bodies in a temperature range from 500.degree. to 1,100.degree. C., but these applications are directed to sintered bodies only which have not undergone a machining treatment. However, they have no investigations of sintered bodies having undergone machining as in the present invention.